Methods are known in which a plastics film is first coated over a large area, is then wound onto rolls for the purpose of transportation or storage, and is subsequently brought into the desired final form on site. Such a procedure would be of particular interest if the surface already exhibited the required properties, such as fastness and appearance, directly after forming without further coatings. This concept offers great potential in the manufacture, for example, of add-on parts for motor vehicles by plastics processors, where the more complex step of coating three-dimensional components could be replaced by the more simple coating of a flat substrate. In addition, by using uniform coated films it is possible to avoid the problem, which occurs frequently in the separate coating (so-called offline coating) of add-on parts for motor vehicles, that the colour of the respective surface-coating layers is not identical (colour matching).
In general, good surface properties require a high crosslinking density of the coating. However, high crosslinking densities lead to duromeric behaviour with maximum possible degrees of stretch of only a few percent, so that cracks tend to form in the coating during the forming operation. This obvious conflict between the required high crosslinking density and the desired high degree of stretch can be resolved in different ways, for example by carrying out curing in two steps, before and after forming.
This can be effected, for example, by drying/curing according to two different mechanisms.
EP-A 0 819 516 describes a method of coating an object during a forming operation by means of a formable, radiation-curable coated film. This method has the disadvantage that, owing to the low glass transition temperature, the coated film does not have adequate block resistance before forming and after-curing. This impairs considerably the handling thereof prior to final curing and is a major disadvantage for industrial application because, for example, such films cannot be rolled up or can be rolled up only with the use of protective films, because otherwise they stick together. In addition, apart from the glass transition temperature and the naming of polymer classes (“phosphazenes, urethanes, acrylates”), this prior art does not indicate what properties the components of a surface-coating system should have in order to permit thermoplastic formability and duromeric behaviour, in particular fastness to weathering and scratching after final curing. In addition, there is no mention of the degrees of stretch that can be achieved.
WO 00/63015 likewise describes a coated formable film which can be cured by means of radiation. Improved block resistance prior to forming is achieved by the addition of polymeric components having a glass transition temperature above 40° C. Although two-stage curing is mentioned (“Moreover, the radiation-curable composition can comprise, in addition to radiation-curable compounds, also compounds that contribute to curing by other chemical reactions”), no reproducible description is given of how such systems can be prepared. Furthermore: In the preparative description there is an inconsistency between the preparation temperature of the radiation-curable coating in the melt of the polymeric component “at 160° C.” and the thermal curing which is later to be carried out on the substrate at “up to 150° C., preferably up to 130° C.”.
In summary, it is found that the prior art does not disclose surface-coating systems for the coating of a post-formable film that fulfil the following requirements:    1) simple application by conventional methods to a film or a film composite,    2) thermal curing via a polyaddition mechanism which results in a block-resistant thermoplastic coated film that can be post-formed using appropriate tools,    3) final curing of the surface coating on the formed, coated film by radiation, the fastness properties of the coating that are achieved being comparable with those which can be obtained by conventional surface coating of already formed objects.